Over the years I have often wondered, "why do juniors hit the ball hard most of the time?" After teaching and watching junior tennis for so long, I have decided that the answer is two fold, "it's easy" and "it requires less thinking!"
With modern technology, young players are able to create great racket head speed and power. Unfortunately, this usually happens before control, technique, placement, consistency and shot selections are learned. Consequently, the majority of juniors quit thinking. They mindlessly go through the motions of competing by just "whaling" on the ball and going for winners. They often, upon exiting a losing tennis match, describe in detail how many aces or winners they hit, of course, not remembering how many unforced errors they made or how many shots they over hit.
If the only thing you do when you practice is hit the ball hard that is what will happen when you play. Remember when you practice or take lessons, start working on all of your shots. You might not like them all, or you might not be good at all of them in the beginning, but keep practicing them.
Then the next time you play a match, try mixing up your shots, changing the height of your shots above the net, and changing the spin and speed of your shots. This will keep your opponent off guard. Early in the match take note of which of your shots irritate your opponent the most -- it may not be your famous "smoking gun" shot. Now when you do hit the ball hard it will be done with the purpose of giving your opponent a change up, thus you will not feel like you must hit the ball hard all the time. Ask yourself sometimes, "how soft can I hit the ball and still win the point?" Then underspins, short angles, lobs and drop shots will start developing. The use of these shots will keep your opponents even more off balance while he or she is expecting to receive your hard shots. Continue to hit the ball hard, but only sporadically. Tennis is still a thinking game, so play smart.
Good luck on the courts!